1. Field of the Invention
The present invention relates to compact lens systems for use in copying machines
2. Description of the Related Art
To provide copying machines of reduced body size, it has been strongly desired in recent years to make the copying lens system also smaller-sized and wider in the angle of view. The lens systems heretofore known for use in copying machines include, for example, those of the Topogon type which are four-component lens systems with a wide angle of view as disclosed in Japanese Laid-Open Patent Applications No. 59-90811 and No. 59-45418. However, these lens systems are generally great in overall length, i.e., as great as at least 0.3 in the ratio of the overall length of lens system to the focal length, .SIGMA.d/f, and therefore they still remain to be improved in providing compactness.
The specifications of U.S. Pat. Nos. 3,370,904, 3,507,560 and 3,731,990 disclose wide-angle lens systems which are about 60 degrees in the angle of view 2.omega. but are as great as 11 to 16 in f-number. The specification of U.S. Pat. No. 2,031,792 discloses a wide-angle lens system which has an angle of view 2.omega. of 100 degrees and an f-number of 6.3 but which is great in overall length, i.e., 0.35 in the value of .SIGMA.d/f, and difficult to produce because of very small lens thicknesses.
The specification of U.S. Pat. No. 3,449,040 further discloses a lens system having an angle of view 2.omega. of 66 degrees and an f-number of 5.6 but is 0.44 in the value of .SIGMA.d/f, hence a great overall length
Such symmetrical lens systems can be of a wide angle of view but are usually limited to about 8 in fnumber and are relatively large-sized since the overall length is also limited to about 0.3 in term of .SIGMA.d/f.